Improve generation time: do not check table entries for constness
Checking all table entries for constness (in the symbolic sense, by sympy) is quite expensive. With the new backend, generating a P2 noopts operator now only takes about 1/3 the time (a drop by 40s). With the old backend, the relative difference is not quite as dramatic but we won't be using that for much longer.
Edited by Daniel Bauer